1. Field of the Invention
The present invention generally relates to soldering techniques, and in particular to a soldering method for a nickel/gold electroless plated surface. The present invention can be applied to a wiring structure, a circuit, and a method for manufacturing the same.
2. Description of the Related Art
Soldering is used to not only electrically connect an electronic device to a printed circuit substrate but also mechanically fix it to the printed circuit substrate. Therefore, to ensure a sufficient strength at a solder joint is one of the important issues in the field of mounting techniques.
In the case of an electronic device of relatively large in size, the electrode area thereof to be soldered, that is, solder-contact area, is also relatively large and therefore the strength of a solder joint is not a significant issue in design. Since a mechanically sufficient solder joint strength is achieved in the case of a relatively large sized device, Sn—Pb eutectic solder has been widely used because of an easy-to-handle material with a low melting point. It should be noted that Sn—Pb eutectic solder itself does not have an excellent mechanical strength.
However, in the case of an electronic device of very small size, such as CSP (Chip Size Package), the electrode area thereof to be soldered, that is, solder-contact area, is also very small. Needless to say, the area of a pad on a printed circuit substrate must also be reduced so as to fit with the external electrodes of a CSP-type device.
For example, in the case where a BGA(Ball Grid Array)-type package with 0.8 mm pitches and each solder ball approximately 0.5 mm in diameter is mounted on a printed circuit substrate, it is necessary to reduce the diameter of a pad on the printed circuit substrate to 0.4 mm. The area of such a pad 0.4 mm in diameter is only a fraction of the area of a pad on a printed circuit substrate to be mounted with a conventional 0.5 mm-pitch QFP (Quad Flat Package).
The more an electronic device is miniaturized, the smaller the electrode area of a contact area to be soldered, that is, solder-contact area. Therefore, the strength of a solder joint becomes a significant issue in design.
In general, a wiring conductor is made of copper (Cu) or aluminum (Al). However, it cannot be said that copper and aluminum essentially lend themselves to use for soldering and further these materials may rust before soldering. To make the to-be-soldered surface of a wiring conductor suitable for soldering and prevent it from rust, therefore, surface treatment such as solder plating or, in these years, nickel/gold plating, is usually made on the necessary surface of the wiring conductor.
Since electroplating needs special wires for electroplating on a package and a printed circuit substrate, the recent trend is to replace electroplating with electroless plating. As electroless plating, in general, nickel-phosphorus plating is employed for nickel plating. Hereafter, a nickel/gold-plated layer, in which a nickel layer is formed by nickel-phosphorus electroless plating, is called “nickel/gold electroless plated layer”.
The Inventors, however, found that a nickel/gold electroless plated layer has less mechanical strength. For example, in the case where a CSP device with 0.8 mm pitches and each Sn—Pb eutectic solder ball being 0.5 mm in diameter is soldered to respective nickel/gold electroless plated pads on a printed circuit substrate, it is observed that the solder is peeled from the boundary surface of the nickel layer.
As one of protection methods to avoid such destructive peeling phenomenon, a space between the CSP device and the printed circuit substrate is filled with an adhesive. However, this method is costly.
As another protection method, a larger soldered area is intentionally formed to increase in mechanical strength at a solder joint. For example, large size reinforcing electrodes are formed in advance and, by soldering these reinforcing electrodes, sufficient mechanical strength can be obtained. However, these large size reinforcing electrodes blocks miniaturization, which is the most important feature of CSP.
As still another protection method, it is considered that the solder plating is employed in place of the nickel/gold electroless plating. However, the solder plating provides less smoothness after plating, resulting in unstable mounting of parts. This may cause manufacturing yield to be reduced. Such a disadvantage is more significant because the electrode area of a contact area to be soldered becomes smaller with recent advances in miniaturization.
As further another protection method, it is considered that a coating of organic compound is employed. However, an organic compound coating cannot prevent the conductor from rust for a sufficiently long time.
As described above, as of now, freedom of choice is limited to selecting the nickel/gold electroless plating.